The biggest difference between ERW steel pipe and seamless steel tube is that ERW has a weld seam, which is the key to the quality of ERW steel pipe. Modern ERW steel pipe production technology and equipment, due to years of unremitting efforts in the world, especially the United States, have made the seamless ERW steel pipe has been relatively satisfactory.
Some people divide the seamlessness of ERW steel pipes into geometric seamlessness and physical seamlessness.
Geometric seamless is to remove the internal and external burrs of the ERW steel pipe. Due to the continuous improvement and improvement of the structure of the internal burr removal system and the cutter, the removal of internal burrs in steel pipes of large and medium diameters has been better treated. The internal burr can be controlled from -0.2mm to 0.5mm.
Physical seamless means that there is a difference between the metallographic structure inside the weld and the base metal, which causes the mechanical properties of the weld area to decrease, and measures need to be taken to make it uniform and consistent. The high-frequency welding thermal process of ERW steel pipe caused the temperature distribution gradient near the edge of the tube blank, and formed characteristic regions such as melting zone, semi-melting zone, overheating structure, normalizing zone, incomplete normalizing zone, and tempering zone. The microstructure in the superheated zone has a sharp austenite grain growth due to the welding temperature above 1000 ° C, and a hard and brittle coarse crystal phase will be formed under cooling conditions. In addition, the existence of a temperature gradient will generate welding stress. In this way, a situation where the mechanical properties of the weld area are lower than the base material is formed. The physical seamlessness is to use the intermediate frequency induction heating device to heat the weld area to AC3 through the conventional local heat treatment process of the weld.
(927 ° C), and then an air cooling process with a length of 60 m and a speed of 20 m / min, and water cooling if necessary. The use of this method has achieved the purpose of eliminating stress, softening and refining the structure, and improving the comprehensive mechanical properties of the welding heat affected zone. So far, the world's advanced ERW units have generally adopted this method to process welds and have achieved good results. High-quality ERW steel pipes not only cannot identify the weld, but also have a weld coefficient of 1, matching the structure of the weld zone to the base metal.
Because ERW steel pipe uses hot rolled coil as raw material, the wall thickness can be controlled uniformly at about ± 0.2mm. Both ends of the steel pipe are in accordance with the American APl standard or GB / T9711.1 standard, the bevel is repaired at the end, and the fixed length is delivered. In recent years, various natural gas pipeline network projects and gas companies have widely adopted ERW steel pipes as the main steel pipes of urban pipeline networks.
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